The present invention relates generally to the design and fabrication of a lightweight titanium member having stiffness and temperature characteristics that could allow the design of lighter, simpler and more cost effective mechanical arrangements. More particularly, the present invention has one form wherein a high performance shaft is formed of a titanium alloy tube having a gamma titanium aluminide alloy core metallurgically bonded therein. This high performance hybrid shaft has significantly increased stiffness over conventional titanium alloy shafts with no effective increase in density. Although the invention was developed for use in a gas turbine engine, certain applications may be outside of this field.
It is well known that a gas turbine engine integrates a compressor and a turbine that have components that rotate at extremely high speeds and the components are generally subjected to elevated temperatures. The rotatable components are coupled to shafts which are also subjected to the temperatures and loading inherent to most gas turbine engines. The necessity for reliable and durable shafts has resulted in the utilization of steel and nickel based alloy materials. In spite of their relatively high density and the cost and difficulty associated in manufacturing shafts from these materials they have been utilized extensively to produce gas turbine engine shafts. However, gas turbine engine designers continuously strive to decrease the weight of rotating gas turbine engine components which provides many benefits including a reduction in the centrifugal forces generated by these components. Further, the reduction in weight of the components in an aircraft gas turbine engine reduces the power consumed in the propulsion of these components.
Designers of gas turbine engines are fully cognizant that one technique for reducing the weight of gas turbine components is to use lower density materials. As a result, the designers of gas turbine engine shaft systems have become interested in titanium alloys in order to produce a relatively lightweight shaft design. However, an inherent limitation with the use of titanium alloys for many designs is the relatively low stiffness which traditional titanium alloys exhibit.
The relatively recent development of titanium alloys based on an intermetallic compound, gamma titanium aluminide (TiAl), having good high temperature (to about 1600.degree. F.) properties makes them ideal candidates for high temperature components within a gas turbine engine. This is because they exhibit improved stiffness characteristics. Further, because of their high strength to weight ratios, gamma titanium aluminides may be used to fabricate parts having substantially lighter weight then parts fabricated from steel or nickel based super alloy parts.
Although gamma titanium aluminides have material properties that are very desirable from a component design standpoint, they are brittle at room temperature and are considered generally hard to fabricate. For many applications involving gamma titanium aluminides, either fusion welding or brazing would probably be the most desirable techniques for joining the gamma titanium aluminide details to a similar material or to other high temperature materials. However, existing welding and brazing techniques have proved unsatisfactory in many respects for providing reliable high temperature joints for titanium aluminide alloys. Therefore, a need exists for a fabrication technique that can lead to a lightweight high stiffness shaft that can utilize the material characteristics of titanium aluminide in combination with dissimilar material combinations such as steel, nickel or conventional titanium where there unique toughness and hardness characteristics might be dictated by a multi-alloy fabricated system by design requirements.
Although the migration from contemporary steel and nickel based shafts to titanium alloy shafts is a step in the right direction, the need for additional improvement in producing a lightweight high stiffness shaft still remains. The present invention satisfies this need in a novel and unobvious way.